University of Crete The JavaTM Memory Model (JMM) Foivos Zakkak 27th of March 2017 

27/03/2017 2 of 14 Foivos Zakkak Introduction • Multi-core processors create multiple copies of data on their cores’ caches to improve performance • Maintaining these copies consistent is not trivial • Trade-off between performance and consistency ● Usually we are only interested about data we can observe 

27/03/2017 3 of 14 Foivos Zakkak Introduction Different architectures feature different memory models Type ARMv7 and Power X86 AMD64 Loads reordered after loads    Loads reordered after stores    Stores reordered after stores    Stores reordered after loads    Atomic reordered with loads    Atomic reordered with stores    Dependent loads reordered    Incoherent instruction cache pipeline    Source: https://en.wikipedia.org/wiki/Memory_ordering 

27/03/2017 4 of 14 Foivos Zakkak Introduction • Some programming languages define their memory models  JavaTM  C++11  Unified Parallel C (UPC) • Why?  To provide consistent behavior across different platforms 

27/03/2017 5 of 14 Foivos Zakkak What is a Memory Model ?  Contract between language or processor designers and developers  Helps language implementers build the runtime and/or compiler  Helps developers understand a program’s behavior 

27/03/2017 6 of 14 Foivos Zakkak The JavaTM Memory Model  Guarantees sequential consistency in data-race-free (DRF) programs  “[...] the result of any execution is the same as if the operations of all the processors were executed in some sequential order, and the operations of each individual processor appear in this sequence in the order specified by its program.” – L. Lamport  No out of thin air values under any circumstances  Reads may only observe values written by some write acting on the corresponding variable  Monitors provide mutual exclusion 

27/03/2017 7 of 14 Foivos Zakkak The JavaTM Memory Model Approach • Model all legal executions (in algebraic way) • Map instructions to actions e.g. a=4 maps to a write action • Synchronization actions ● Release-Acquire pairs ● Implicit vs Explicit 

27/03/2017 8 of 14 Foivos Zakkak Release-Acquire pairs • Thread finish / Thread join – Explicit • Monitor.wait() / Monitor.notify() – Explicit • Monitor exit / Monitor enter – Implicit • Thread.start() / Thread.run() – Implicit • … Reads following an acquire action must be able to observe writes performed before the corresponding release action 

27/03/2017 9 of 14 Foivos Zakkak Examples ● Implicit synchronized (this) { b = 3; } … synchronized (this) { a = b; } ● Explicit b.notifyall(); … b.wait(); … t.join(); 

27/03/2017 10 of 14 Foivos Zakkak Formal Definition  Action kinds  read, write, wait, join, etc.  Grouped in regular and synchronization actions  Total orders  Synchronization order  Partial orders  Program order  Synchronizes-with order  Happens-before order  Well-formedness conditions 

27/03/2017 11 of 14 Foivos Zakkak Actions ordering visualization 

27/03/2017 12 of 14 Foivos Zakkak Well-formedness conditions  Each read of a variable sees a write to this variable  All reads and writes of volatile variable are volatile actions  The number of synchronization actions preceeding another synchronization action is finite  Synchronization order is consistent with the program order  Lock operations (monitors) are consistent with mutual exclusion  The execution obeys synchronization order consistency  The execution obeys happens-before order consistency  Every thread’s start action happens before its other actions except for initialization actions 

27/03/2017 13 of 14 Foivos Zakkak The JavaTM Memory Model Limitations • Defined by observing existing JVMs • Tailored after shared memory architectures • Not machine checkable • Non-intuitive definition 

27/03/2017 14 of 14 Foivos Zakkak Better Understanding For in depth understanding of JMM refer to the following resources: • The JSR-133 Cookbook for Compiler Writers • JSR-133 Java Memory Model and Thread Specification • SPECIAL POPL ISSUE: The Java Memory Model (not published) • Jeremy Manson's Ph.D. Thesis